[PATCH] usb: misc: legousbtower: Fix NULL pointer deference

[James]

This patch fixes a NULL pointer dereference caused by a race codition 
in the probe function of the legousbtower driver. It re-structures the 
probe function to only register the interface after successfully 
reading the board's firmware ID.

The probe function does not deregister the usb interface after an error
receiving the devices firmware ID. The device file registered
(/dev/usb/legousbtower%d) may be read/written globally before the probe
function returns. When tower_delete is called in the probe function
(after an r/w has been initiated), core dev structures are deleted while
the file operation functions are still running. If the 0 address is 
mappable on the machine, this vulnerability can be used to create a 
Local Priviege Escalation exploit via a write-what-where condition by 
remapping dev->interrupt_out_buffer in tower_write. A forged USB 
device and local program execution would be required for LPE. The USB
device would have to delay the control message in tower_probe and accept
the control urb in tower_open whilst guest code initiated a write to the 
device file as tower_delete is called from the error in tower_probe.

This bug has existed since 2003. Patch tested by emulated device.

Signed-off-by: James Patrick-Evans <james@xxxxxxxxx>
---
drivers/usb/misc/legousbtower.c | 35 +++++++++++++++++------------------
1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/drivers/usb/misc/legousbtower.c b/drivers/usb/misc/legousbtower.c
index 7771be3..4dd531a 100644
--- a/drivers/usb/misc/legousbtower.c
+++ b/drivers/usb/misc/legousbtower.c
@@ -898,24 +898,6 @@ static int tower_probe (struct usb_interface *interface, const struct usb_device
       dev->interrupt_in_interval = interrupt_in_interval ? interrupt_in_interval : dev->interrupt_in_endpoint->bInterval;
       dev->interrupt_out_interval = interrupt_out_interval ? interrupt_out_interval : dev->interrupt_out_endpoint->bInterval;

-       /* we can register the device now, as it is ready */
-       usb_set_intfdata (interface, dev);
-
-       retval = usb_register_dev (interface, &tower_class);
-
-       if (retval) {
-               /* something prevented us from registering this driver */
-               dev_err(idev, "Not able to get a minor for this device.\n");
-               usb_set_intfdata (interface, NULL);
-               goto error;
-       }
-       dev->minor = interface->minor;
-
-       /* let the user know what node this device is now attached to */
-       dev_info(&interface->dev, "LEGO USB Tower #%d now attached to major "
-                "%d minor %d\n", (dev->minor - LEGO_USB_TOWER_MINOR_BASE),
-                USB_MAJOR, dev->minor);
-
       /* get the firmware version and log it */
       result = usb_control_msg (udev,
                                 usb_rcvctrlpipe(udev, 0),
@@ -936,6 +918,23 @@ static int tower_probe (struct usb_interface *interface, const struct usb_device
                get_version_reply.minor,
                le16_to_cpu(get_version_reply.build_no));

+       /* we can register the device now, as it is ready */
+       usb_set_intfdata (interface, dev);
+
+       retval = usb_register_dev (interface, &tower_class);
+
+       if (retval) {
+               /* something prevented us from registering this driver */
+               dev_err(idev, "Not able to get a minor for this device.\n");
+               usb_set_intfdata (interface, NULL);
+               goto error;
+       }
+       dev->minor = interface->minor;
+
+       /* let the user know what node this device is now attached to */
+       dev_info(&interface->dev, "LEGO USB Tower #%d now attached to major "
+                "%d minor %d\n", (dev->minor - LEGO_USB_TOWER_MINOR_BASE),
+                USB_MAJOR, dev->minor);

exit:
       return retval;
--

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